The present invention relates generally to voltage controlled oscillators used in frequency synthesizers in communication devices and, in particular, to a dual band voltage controlled oscillator with low phase noise.
Analog and digital devices for personal communications such as in portable telephones operate in more than one frequency band. The frequency bands around 1.0 GHz and 2.0 GHz are now used in the majority of portable phones. Local oscillators for these dual band portable telephones are required to operate in two widely separated frequency ranges. Stringent requirements are placed on these oscillators in terms of their operating performance parameters. The oscillator characteristics such as phase noise, load pull, power supply pushing and output power must meet the specified requirements. At the same time, the oscillator must be cost effective and readily manufacturable. Prior art dual band devices have included: completely separate local oscillators with combiner networks, local oscillators with doublers or triplers to multiply frequency, PIN diodes or other similar RF switching devices, and extremely wide band oscillators having an operable frequency range that overlaps the two bands of interest.
The use of signal frequency doublers or triplers in oscillators to multiply the frequency has disadvantages in that spurious signals are always present in the output. These spurious signals must be filtered out to avoid degrading receiver performance or interference with other radio services. In addition, the parts count increases greatly with doublers and triplers and the desired output frequencies must exactly match those multiples. Phase noise increases with frequency doubling or tripling.
Some oscillators use PIN diodes. The disadvantage of PIN diodes in oscillators is that PIN diodes require significant DC current to obtain a low xe2x80x9conxe2x80x9d impedance, and when the PIN diodes are xe2x80x9coffxe2x80x9d they can create high levels of harmonically related spurious signals, losses and distortions. Tank circuits associated with the PIN diodes reduce circuit Q, which reduces efficiency, and causes higher phase noise in the output circuit.
The field of oscillators includes wide band oscillators and dual band voltage controlled oscillators (VCO). The disadvantage of using extremely wide band oscillators is that wideband oscillators are necessarily very sensitive to tuning control. This sensitivity makes the oscillator more susceptible to noise on the tuning control line. Correspondingly, more sensitive tuning requires tighter coupling to the tuning element (varactor) of the oscillator which causes higher losses in the associated tank circuit.
One example of a dual band voltage controlled oscillator (VCO) is shown in U.S. Pat. No. 5,821,820 to Snider. This dual band VCO uses separate oscillators for each frequency band with a combiner circuit, however, only one varactor is required. One problem with this type of oscillator is related to the DC switching of the two transistors. In order to switch the bands, the VCO uses two ENABLE lines. This adds cost and complexity to the circuit design.
Another problem with the teaching of Snider resides in the common use of the same tank circuitry in generating both of two widely separated frequency bands (900 MHz and 1.8 GHz). It is not possible to design the tank in such a way that the conditions for minimum phase noise are met simultaneously in both bands.
Snider draws those two frequency bands from the collectors of each of two bi-polar junction transistors, one frequency band from each collector. It is noted that extracting the output signals from the collector of the two transistors is not a good choice when harmonics content is an issue. Even though the loaded Q of the tank circuit is relatively high, the current through the active element of an oscillator is distorted in order to sustain the limiting process. Because the collector voltage is proportional to the collector current it is expected that the harmonics content will be high.
There is a present unmet need for a dual band voltage controlled oscillator with low phase noise that overcomes the limitations of the prior art and is readily manufacturable at a low cost.
It is a feature of the invention to provide a dual band voltage controlled oscillator with low phase noise.
A further feature of the invention is to provide a multi-band oscillator that includes a high and low tank circuit that is responsive to a tuning voltage, each tank circuit having an output. A high voltage controlled oscillator is operated at a high frequency and has an input and an output. The output of the high tank circuit is coupled to the input of the high voltage controlled oscillator. A low voltage controlled oscillator operates at a low frequency and has an input and an output. The output of the low tank circuit is coupled to the input of the low voltage controlled oscillator. A combiner circuit has a high and low input and an output. The high and low inputs of the combiner circuit are coupled to the respective outputs of the high and low voltage controlled oscillators. A low frequency bandstop filter is coupled to the output of the high frequency voltage controlled oscillator. The low frequency bandstop filter operates so as to prevent the low frequency signal from being diverted to ground rather than produced as an output from the combiner.
Those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the design of other structures, methods and systems for carrying out the several purposes of the present invention.